This application is a 35 USC 371 of PCT/SE99/02293 filed Dec. 8, 1999.
The invention relates to a method for manufacturing of strips of stainless steel, comprising rolling in cold condition of strips which in a foregoing process have been manufactured through strip casting and/or have been hot rolled. The invention also relates to a rolling mill line to be used at the carrying out of the method.
Cold rolling of stainless steel strips is performed for one or several purposes. The basic purpose is generally to reduce the thickness of the starting strips, which normally have been hot rolled in a foregoing hot rolling line to a thickness of the hot rolled strips, which is not less than 1.5 mm and normally is in the order of 2-4 mm, but can be up to 6 mm. Conventionally, initial annealing, cooling, and descaling shot-blasting as well as pickling in one or more steps precede the cold rolling, for the achievement of a starting material for the cold rolling without oxides and scale residues from the foregoing hot rolling. As an alternative the hot rolling can completely or partly be replaced by manufacturing of strips through casting, which strips may have a thickness down to what is normal for hot rolled strips or be a few millimetres thicker, but also in this case the cold rolling normally is preceded by initial annealing, cooling, descaling shot-blasting, and pickling, to the extent the technique has been implemented at all. At the cold rolling, which conventionally is carried out in a plurality of consecutive cold rolling operations, possibly alternating with annealing, cooling, descaling, and pickling operations, the thickness can be reduced down to 1 mm and in some cases to even thinner gauges. At the same time it is possible to produce, in these conventional cold rolling mills, strips with a very fine surface, a so called 2B-surface, if the rolling is finished by heat treatment, pickling, and skin-pass-rolling, or even finer if bright annealing is employed. A cold rolling also may have as a main purpose or as an additional purpose to increase the strength of the strip material. For this purpose is has also been suggested, as a complement to cold rollingxe2x80x94EP 0 738 781xe2x80x94to cold stretch the strip subsequent to annealing, so that the strip is plasticised and is elongated permanently, at the same time as its thickness is reduced. Further is it knownxe2x80x94U.S. Pat. No. 5,197,179 andxe2x80x94EP 0 837 147xe2x80x94to perform at least a first cold rolling operation on the cooled hot rolled strip or on the cooled cast strip prior to heat treatment, pickling, and possible further cold rolling operations in order to bring the strip to desired final gauge. It is, however, characteristic for methods and rolling mill lines known so far that they are expensive and/or difficult to adapt to widely disparate requirements as far as strip thickness, surface conditions, and strength of the final product are concerned. This particularly applies when hot rolling and subsequent cold rolling, as well as operations in connection with the hot rolling and the cold rolling are considered as an integrated process of production.
It is a purpose of the invention to attack and solve the above complex of problems. This, according a first aspect of the invention, can be achieved therein that the cold rolling is performed in a rolling mill line, which comprises, in the initial part of the line, at least two initial cold rolling mills in series, after said initial cold rolling mills at least one annealing surface and at least one pickling section, and in a terminating part of the line, at least one more cold rolling mill, that the cast and/or hot rolled strip, which is dark coloured by oxides on the surfaces of the strip, with the dark coloured oxides remaining on the surfaces of the strip, first is cold rolled in at least one of said initial cold rolling mills so that the thickness of the strip is reduced totally by 10-75%, that the strip then is annealed and pickled in said annealing and pickling sections and is cold rolled in said at least some more cold rolling mill so that its thickness is reduced by 2-20%, that the strip then is fed once more in the same direction through the same rolling mill line, wherein the strip is rolled again in at least one of said initial cold rolling mills so that the strip consecutively is cold rolled in at least one of said more cold rolling mills and in at least one of said initial cold rolling mills, comprising cold rolling in at least three cold rolling mills without intermediate annealing, reducing the thickness by totally 30-75% before the strip again is annealed and pickled. When the strip passes for the second time through the rolling mill line, the strip is preferably rolled again in one of said more cold rolling mills in the terminating part of the rolling mill line, but is this time only skin-pass-rolled, reducing the strip thickness by 0.2-1.5%.
As an alternative, any rolling in the terminating part of the line is excluded as the strip is being passed through the line for the first time, and this is particularly the case if only a skin-pass-rolling mill is provided there, in which case the strip is subjected to rolling in at least three consecutive cold rolling mills in the initial part of the line, as the strip a second time is caused to pass through the line, the strip being reduced by totally 30-75% before the strip again is annealed and pickled and possibly skin-pass-rolled.
The method which has been described above makes manufacturing of strips with very fine surface possible. In order to achieve such a surface it is, however, important that the strip is subjected to descaling prior to pickling, and that such descaling is performed in such a mode that the surfaces are not impaired. Conventionally, descaling is carried out though powerful shot-blasting in one or more steps, a treatment which however results in the undesired damages of the strip surfaces. According to an aspect of the invention, the descaling instead is carried out by bending the strip several times in different directions about rolls, at the same time as the strip is cold-stretched so that it is permanently elongated 2-10% prior to pickling according to a technique which is known per se through said EP 0 738 781. Through this treatment an efficient descaling is achieved without impairing the strip surfaces. This descaling can be completed with a mild shot-blasting, which can be performed before or after the descaling, preferably before aiming at removing only loose oxides in order, through accumulation of oxides, not do disturb subsequent descaling. If the shot-blasting is carried out subsequent to the descaling it is correspondingly achieved that loose oxides are removed, the shot-blasting in each case being carried out in such a mild way that the metallic surfaces of the strip are not impaired.
According to the above described first aspect of the invention, the strip passes twice through the cold rolling mill line. According to another aspect of the invention, this possibility is not utilised in the manufacturing of strips, when the aimed results in the first place are to provide a final product having a high yield strength and surfaces which are fine, even though they do not satisfy the requirements of 2B-quality. According to this aspect of the invention, the invention is characterised in that the cold rolling is performed in a rolling mill line, which comprises, in the initial part of the line, at least two initial cold rolling mills in series, after said initial cold rolling mills at least one annealing section and at least one pickling section, and in a terminating part of the line, at least one more cold rolling mill, that the cast and/or hot rolled strip, which is dark coloured by oxides on the surfaces of the strip, with the dark coloured oxides remaining on the surfaces of the strip, first is cold rolled in at least one of said initial cold rolling mills so that the thickness of the strip is reduced by totally 10-75%, that it then is annealed in at least one annealing furnace in an annealing section, that it subsequent to annealing and rolling is subjected to descaling in at least one descaling unit in which the strip is bent several times in different directions about rollers at the same time as the strip is cold-stretched, so that the strip is permanently elongated 2-10%, wherein the scales are caused to be broken, that the strip then is pickled, and that the pickled strip finally is cold rolled in a non-lubricated condition in said at least one more cold rolling mill reducing the thickness by 2-20%.
As has been mentioned in the foregoing description of the background of the invention, it is conventional to hot roll strips to a final hot rolled strip gauge of 2-4 mm and it may even occur that hot rolling is made all the way down to 1.5 mm. The most complicated part of the hot rolling is the final part, i.e. when one is operating with considerably thin strips. This phase is difficult to control and there are also produced much oxides on the strips in relation to the strip thickness. Further, the yield in production in the hot rolling mill is reduced the more the strip thickness is reduced. In order further to improve the starting material used for the subsequent cold rolling it is also advantageous to quench-cool the strip from the final rolling temperature down to below 500xc2x0 C. in order on one hand to produce as thin oxides layers as possible and on the other hand to avoid precipitation of grain boundary carbides in the surface layers. In accordance with another aspect of the invention it is the purpose to integrate the initial hot rolling and treatment of the strip in connection with the hot rolling with the subsequent cold rolling in such a mode that there is achieved a good production economy from an overall point of view, with an improved capacity in the hot rolling mill including less risk of bottle necks in the hot rolling mill line, as well as a final product after cold rolling which can satisfy high requirements as far as good quality is concerned. According to this aspect of the invention, the invention relates to a method for manufacturing strips of stainless steel, comprising hot rolling in an initial process and subsequently cold rolling in a rolling mill line, characterised in that the hot rolling is stopped when the strip thickness has been reduced to a thickness between 2.5 and 6 mm, preferably to between 3 and 5 mm, that the thus hot rolled strip is cooled from the final hot rolling temperature through quenching at a cooling rate of at least 15xc2x0 C./s to below 500xc2x0 C., that it at the subsequent cold rolling is passed twice in the same direction through said cold rolling line which comprises at least two cold rolling mills in the initial part of the line, and after said initial cold rolling mills at least one annealing section and at least one pickling section, said strip, as it for the first time is passing the at least two cold rolling mills in the initial part of the line, being rolled with the dark coloured oxides which the strip has obtained in the hot condition of the strip during the initial process.
At the initial cold rolling of the stainless steel, when the dark coloured oxidic coatings on both sides of the strip steel are there, which oxidic coatings have been formed in connection with the initial process in the hot state of the steel, a crackling of the oxide scales will occur to some degree. This can be considered as an initial descaling operation, which can facilitate the efficient descaling that is performed later, after the annealing, before the strip is pickled. In order that the said initial crackling shall be possible to be utilised efficiently in order to facilitate later descaling and pickling it is desirable that it as far as possible is not eliminated in connection with the annealing, i.e. so that fissures or cracks in the oxide layers do not heal up at the annealing. According to still another aspect of the invention it is a purpose to avoid that effect, i.e. to a substantial degree preserve the initiated braking up of the oxide scales which is achieved through the initial cold rolling on the oxidic surface layers. According to this aspect of the invention, the invention is characterised in that in that the cast and/or hot rolled strip, which is dark coloured by oxides on the surfaces of the strips, remaining from the foregoing manufacturing of the said cast and/or hot rolled strip, is cold rolled in one ore more consecutive cold rolling passes reducing the strip thickness by 10-75% and crackling the oxide scales, i.e. so that cracks are produced in the oxide scales, that the strip then is annealed in a furnace having a furnace atmosphere which contains max 10 vol-% oxygen, preferably max 6 vol-% oxygen, and that the strip thereafter is pickled. The said furnace atmosphere can be obtained e.g. through the technique which is disclosed in WO95/24509, the content which herewith is incorporated in this text by reference. Typically the descaling is finished subsequent to annealing in a mode that has been described in the foregoing, i.e. through cold stretching in connection with bending the strip repeatedly about rolls and without surface destroying shot blasting.
Further characteristics and aspects of the invention will be apparent from the accompanying patent claims and from the following description of said rolling mill line and of how the invention can be reduced to practice according to a preferred embodiment.